StressingOut:Bitcoin“StressTes2ng”

KhaledBaqer,DannyYuxingHuang,DamonMcCoyandNickWeaver

(givenbyRossAnderson)

Overview

•  SpamaNackonBitcoin(July2015)•  Importance:blocksizedebate!AtpresentyoucanbringdownBitcoinfor$50k

•  Goalhere:highlightspammo2fs,inves2gateimpact,analysesecurityeconomics(feeincomeincrease,costofspamcampaign)

•  Usedaclusteringmethodtogrouptransac2onsandfindpaNerns

Bitcoinspam

•  Fan-in:Transac2onsthatabsorbalotofinputsreducetheunspenttransac2onoutput(UTXO)setbuts2lloccupysubstan2alspaceintheblocks

•  Fan-out:Transac2onsthatsplitafewinputsintomanyoutputsoccupyspaceinblocksandalsoincreasetheUTXOset(Mempoolimpact)

•  Dustoutput:“Dust”outputsconveyatrivialamountofvaluebutoccupythesameamountofresourcesintheBitcoinnetwork

Spamcampaign(July2015)

•  Someonesaid,‘stresstes2ngBitcoinnetwork’•  Mo2va2on(?):showBitcoinisvulnerabletoDoS,andgetsupporttoraisetheblocksize

•  DoS:sendtransac2onswithhigherfeestodepletespaceinblocks

•  Spamalsousesmanysimilartransac2onstohaveasignificantimpactonthenetwork

•  Butwhatdoesspamlooklike?

k-meansClustering

k-meansClustering(II)

ImpactonBitcoin:Thenumberoftransac2onsperdayintheblockchain

ImpactonBitcoin(II):Theaveragenumberofunconfirmedtransac2ons

ImpactonBitcoin(III):Thetotalamountoftransac2onfeeseveryday

ImpactonBitcoin(IV):Averagetransac2ondelay

ImpactonBitcoin(V):Averagetransac2onfeespertransac2onperday(normalized)

Summaryofresults(10-dayspamcampaign)

•  385,256(23.41%)outof1,645,667totalBitcointransac2onswerespam

•  Spamincreasedaveragefeesby51%(from45to68Satoshis/byte)

•  Spamincreasedprocessingdelayby72mes(from0.33to2.67hours)

•  CostofthisaNackonBitcoin:$49K,abouthalfbeingtopayhigherfees

Ques2ons?

•  Ifskypedoesn’twork,.emailhardques2onstokhaled.baqer@cl.cam.ac.uk(whocouldnotgetavisaprocessedin2me)

•  Furtherpossiblediscussionsforthebarbecue:sovereignriskandotheremergentproblemsofgovernance